Your comments

Make sure your keyboard is clean. If you have dropped a jam sandwich on it, the buttons may be sticky. Any problem solving should always start by removing the easiest possible solutions first before you get all technical.
Some maps and modes are better than others to gain xp. I use arena CP. This is the fastest way to get xp I have discovered. I can get 3000 xp in an hour without a multiplier pass. I tend to use mammouth with either firebird, thunder or twins for this. So with a double multiplier pass that would be 6000xp in an hour. - Generalissimo in no time!
I do use isida sometimes and its OK for getting xp, but I don't rate it quite as much as some.
Someone on here suggests you get another account on here and play yourself - don't. It is against the #rules and will get you banned.
A couple of weeks ago, I met someone on my team who kept pushing me off the map. At the end of the game I changed teams and spend an hour hunting him down and ignoring everybody else. It worked - we won the game, I got most crystals and the troll used dozens and dozens of supplies to protect himself and only got a few crystals. I haven't seen him again and it still gives me a deep sense of satisfaction knowing that his trolling cost him a lot of crystals.
It is  worth trying this stategy, but be sure they are trolling rather than just being clumsy as a one off.
Any paint that has protection from thunder will protect your tank from your own thunder to the same degree it protects against other thunders. I always use prodigi or emerald when armed with thunder but these are only available to higher  ranks. But there are other options at all ranks after you have reached corporal, and the first one is metallic. I suggest you micro upgrade it as much as you can if you use thunder a lot.
The number of crystals you recieve depends on the xp you have earned in the battle. The number of crystals added to the fund depends on the rank of the tanks you have killed, so there is no easy answer to that. In the team battles I usually play - If you are on the winning side - at the rank of brigadier I usually receive about one crystal per point of xp, or just below. Say 1400 for 1500 xp. On my account that is a staff sergeant it is about 1 crystal for 3 xp. Say 500 crystals for 1500 xp. But as I said, it depends on the rank of the tanks you are killing. Also, if a couple of high scoring tanks drop out of the battle, that means the crystals they would have earned are shared out to the others on the team, proportionate to the xp points each has scored. This will increase crystals per xp, sometimes quite dramatically. On my brigadier account I have, a couple of times, got as high as 2 crystals per xp when this happens! Unfortunately, giving you a more accurate answer would be about as easy as trying to nail down a piece of fog.
Everybody seems to be in the dark here, SO I will have a punt as well. I have 15 accounts, and I sign into all of them daily. (They all end in 1821 if you want to check!). The pro battle pass and double crystal card between them probably account for at least one third of bonuses from what I have seen. The chances of crystals I would put at less than 10% - maybe once a fortnight (i.e. 7%). And the rest will be drug supplies. But - and I stress this - it is random, and on one account it will take a long time for this to average out. You may get a pro battle pass 3 days in a row, or (much less likely) crystals 3 days in a row.
I should point out I have kept no records - these are only my impressions from my accounts. Only tankionline themselves know the algorithm that they are using!
I've always found twins to be a top weapon. I still have twins M1 fully MU'ed (waiting for a sale for M2) and I have used it devastatingly against 4 star generals with mammouth in arena albeit with the odd bit of drugging. It helps to be insanely aggressive and in your face - If this is your style it may be for you. If you prefer guile and stealth - maybe not.